Forming tools for pressure forming of material, such as sheet steel, usually consist of steel, but pressure forming tools of another material or of composite materials are known.
Swedish patent specification No. 222,276 teaches the use of such pressure forming tools which substantially consist of a body of plastics having a preformed shell of sheet metal, which constitutes the forming surface proper. The idea is that, by backing this shell by means of a body of plastics, pressure forming of sheet metal even of greater thickness than that of the metal shell should be possible. It was found, according to the abovementioned patent specification, that it was necessary to protect the plastic body by using a heat-conducting metal bed between the metal shell and the plastic body, and such metal bed was applied to the rear surface of the shell by metallization and formed together with the preformed metal shell into a composite shell of greater thickness than that of the preformed metal shell in order to achieve a shell of sufficient metal material for the necessary heat absorption and dissipation.
This method involves problems as to the preforming of the sheet metal shell, its fragility and its susceptibility to deformation during handling and the metallization process and bonding to the plastic body.
Furthermore, U.S. Pat. No. 3,631,745 teaches a method of forming dies by so-called thermal spraying, i.e. a method of melting a metallic or ceramic material, normally in the form of wire or powder, in a source of heat and spraying the molten material in the form of droplets onto a surface to form a metal shell a layer. According to this method a matrix is produced by shaping or casting of a wooden or plaster pattern into the form of the die and casting an inorganic substrate over the pattern for forming the matrix. The matrix is cured and whereupon separated from the pattern. Then the matrix is preheated, and the metal which is to form the die is applied onto the preheated matrix by thermal spraying while the matrix is maintained at the same temperature as the metal. The matrix with the metal spray-applied is then placed in a pre-heated oven for slow cooling. Because of different contracting rates of the matrix material and the die material, the latter being titanium, the die is released from the matrix by crushing the matrix material e.g. in that it breaks by the stress forces during cooling. The cavity of the metal die so obtained is filled with a material mixture which, after cooling and hardening, forms a backing body imparting to the metal die the requisite stability and strength for use as a forming tool.
This method directed to special tools of titanium demands several die shell production operations comprising preheating in an oven and slow cooling; and in the production of a large but thin metal die, the die is inclined to be deformed at the separation from the matrix and until the filler in the cavity thereof has hardened.
According to another process disclosed by British patent specification No. 854,636 a forming tool is produced by forming a thin skin of metal over the mould surface of a pattern mould by deposition on that surface of a metal, casting an epoxy resin casting composition in the metallised mould, whereby the thin skin of metal becomes adherent to the surface of the casting, and thereafter removing the metallised casting so produced from the pattern mould. According to this British specification No. 854,636 the metal deposited must be chosen and its thickness controlled (limited) to avoid such distortion of the metal skin, which tends to arise when the thickness of deposited metal increases.
Another drawback of the known method is said to reside in that the production of a thick layer of sprayed metal is not feasible because of severe skrinkage stresses.